(1) Field of the Invention
The present invention relates to facilitating the operation of aircraft and particularly to enhancing the in-flight stabilization of rotary wing aircraft. More specifically, the present invention is directed to stability augmentation systems for aircraft and especially to apparatus for automatically changing the gain of such stability augmentation systems. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Priot Art
While not limited thereto in its utility, the present invention has been found to be particularly well suited for use in enhancing the controllability of vertical takeoff aircraft of the rotary wing type. Such aircraft are, of course, generally known in the art as helicopters. It is well known that a helicopter has inherent dynamic instability. Thus, in the absence of automatic flight control apparatus, the maneuvering of a helicopter requires that an exceptional amount of pilot attention be directed to achieving stability about the roll, pitch and yaw axes. The required degree of attention to the control function severely limits the pilot's ability to perform other necessary functions such as navigation and observation.
The operational problems associated with the instability of a helicopter, as briefly discussed above, have led to the design of stability augmentation systems which are intended merely to improve the stability of the aircraft, such stability augmentation systems providing the desired stabilization without altering the pilot's ability to exercise control and, preferably, without varying the "feel" of the control mechanisms such as the collective and cyclic pitch sticks. Thus, stability augmentation systems permit a helicopter to be flown "hands off" for limited periods of time. A stability augmentation system must be contrasted with an automatic pilot; the latter holding the aircraft on a preselected course and altitude at a preselected attitude during steady state flight conditions.
Some of the requirements of a stabilization augmentation system have been briefly discussed above. In addition to the obvious requirements of reliability and capability of producing dynamic stability about all three axes of movement of the aircraft, such stabilization systems must be functional throughout the operative ranges of speed and power of the craft. It is also exceedingly important that a stability augmentation system be capable of producing dynamic stability for all loading conditions; the loading conditions affecting the gross weight and center of gravity of the aircraft.
In the prior art, stability augmentation systems have lacked the ability to identify helicopter gross weight and center of gravity from in-flight measurements thereby permitting the changing of the augmentation system gains with variation in the aircraft loading. Thus, in prior art stability augmentation systems the gains were either fixed, were varied with airspeed, or were varied by means of an adaptive controller. Fixed gains are naturally a compromise and usually produce marginal performance at other than the design flight conditions. Variation of the stabilization system gains with airspeed alone has not proven to be a sufficient solution and adaptive controllers are complex, expensive and somewhat lacking in reliability. To summarize, the inability of prior art helicopter stability augmentation systems to respond to center of gravity and load weight changes has, particularly in combat zones where loads must be placed aboard and removed from the aircraft quickly and without concern for placement, greatly increased the need for pilot attention to the actual flying of the aircraft at the expense of other duties such as navigation and observation and otherwise rendered the actual control of the aircraft more difficult.